1. Field of the Invention
The present invention relates to a display device and a backlight unit for use in the display device and, particularly, to a structure of a flat emission type backlight unit having a line-shaped light source.
2. Description of the Prior Art
The non-light emission type display device such as, for example, a liquid crystal display device is usually equipped with a flat light emission type backlight unit in order to make a display possible even in a place in which there is no external light or a state of an external light is inadequate. A general structure of such liquid crystal display device will be described with reference to FIG. 1.
As shown in FIG. 1, a backlight unit 10 for a liquid crystal display device includes a line-shaped light source 2 such as a cold cathode fluorescent lamp arranged on a side surface of a light guide panel 1 and a light reflection plate 3 for reflecting light from the light source 2 to the light guide panel 1. A reflection sheet 5 is arranged on a rear surface of the light guide panel 1. The line-shaped light source 2 is connected to a connector terminal 32 through lead wires 33. A light collecting optical sheet 41 such as a prism sheet is arranged on the surface side of the light guide panel 1 and a light diffusing optical sheet 42 is provided between the light collecting optical sheet 41 and the light guide panel 1. These components are housed in a housing 6 to form the back light unit 10.
A liquid crystal display panel 11 is provided on the housing 6 and the liquid crystal display device is completed by sandwiching the housing 6 between an upper shield 12 and a lower shield 13 and fixing the shields 12 and 13 to the housing 6. The fixing of the shields 12 and 13 to the housing is performed by using known fixing means such as threads and screws or detents and notches.
FIG. 2 is a cross section of the backlight unit 10, which is of, for example, an edge illuminating type. The light guide panel 1 is formed of a resin material such as acrylic resin having high light transmittance and takes in the form of a flat panel. The line-shaped light source 2 is arranged along a side edge of the light guide panel 1 and the light guide panel 1 functions to randomly reflect light incident on the side edge thereof from the light source 2 within the light guide panel. The light reflection plate 3 having a U-shaped cross section is arranged on an opposite side of the light source 2 to the side facing the side edge of the light guide panel 1 to cover the line-shaped light source 2 such that light from the light source 2 is efficiently introduced into the side edge of the light guide panel 1. The reflection sheet 5 for reflecting back light coming through a lower surface of the light guide panel 1 toward a front, that is, upper surface thereof is arranged on the lower, that is, rear surface of the light guide plate 1. The light diffusing optical sheet 42 and the light collecting optical sheet 41 are laminated on the side of the upper surface of the light guide panel 1 and the assembly is housed in the housing 6.
Each of these optical sheets is as thin as 0.12 mm and a base material thereof is usually polyester resin. Since, in such case, light transmittance of one sheet is about 90%, the sheet has to be made as thin as possible, otherwise, light transmittance thereof is lowered.
When the backlight unit including such thin optical sheets is assembled together with a non-light emitting display device and operated in high temperature and high humidity environment or in low temperature environment, the light collecting and light diffusing optical sheets of the backlight unit are not only thermally expanded but also thermally deformed, resulting in wrinkles or flexion of the optical sheets. Therefore, the uniformity of luminance is degraded. Particularly, in a case where peripheries of the optical sheets are pressed to the light guide panel in the frame type housing 6 and fixed thereto, wrinkle or bending appears in center portions of the sheets, so that the display quality of the display device is degraded. Even if the optical sheets are not pressed by the frame and fixed thereto, it is necessary to provide a large clearance between the frame and the optical sheets in order to avoid interference between the housing frame and the optical sheets. However, it is difficult to provide large clearance in the backlight unit in view of the commercial request of reducing the size and thickness of the display device. Therefore, the problem of degradation of display quality due to thermal deformation of the optical sheets cannot be solved.
Although techniques for compensating for a shortage of strength of a reflecting/deflecting plate and/or a light guide panel are disclosed in, for example, Japanese Patent Application Laid-open No. S59-52217 and Japanese Patent No. 2856276, these techniques are not related to the reinforcement of optical sheets used in a backlight unit, for diffusing light, collecting light and/or reflecting light, as in the present invention.
An object of the present invention is to provide a backlight unit for use in a non-light emitting type display device such as a liquid crystal display device, which backlight unit includes a thin optical sheet as a light diffuser, a light collector or a light reflector and a reinforcing plate stuck on the optical sheet and having light transmittance higher than that of the one sheet and durability against thermal deformation so that luminance of the backlight unit becomes uniform without irregularity in high temperature and high humidity environment.
Another object of the present invention is to provide a display device using the same backlight unit.
According to the present invention, a backlight unit comprises an optical sheet for transmitting light from a light source and giving an optical change to the light, a transparent reinforcing plate in intimate contact with one of main surfaces of the optical sheet and a reflection member for reflecting the light emitted from the light source to the transparent reinforcing plate such that the light from the light source passed through the transparent reinforcing plate passes through the optical sheet.
The transparent reinforcing plate is preferably formed of a material having light transmittance higher than that of the optical sheet and thermal deformation larger than that of the optical sheet.
The optical sheet is a light collecting optical sheet for increasing a front surface luminance of the backlight unit and the backlight unit further comprises a light diffusing optical sheet arranged on a side of a surface of the reinforcing plate opposite to the side of the other surface thereof on which the light collecting optical sheet is arranged, for increasing the uniformity of luminance of a light emitting surface of the backlight unit and a flat light guide panel arranged between the reflecting member and the light diffusing optical sheet for randomly reflecting the incident light from the light source, within the flat light guide plate.
The light source is a line-shaped light source arranged on a side edge of the light guide panel and the backlight unit further comprises a curved light reflecting plate for reflecting light emitted from the light source toward the side edge of the light guide panel.
The reinforcing plate is preferably stuck on the optical sheet by a transparent tackiness agent. The transparent tackiness agent may be acrylic tackiness agent.
In a case where a base material of the optical sheet is polyester resin, the reinforcing plate is preferably formed of either polyethylene naphthalate resin or polycarbonate.
Particularly, light transmittance of the reinforcing plate for light from the light source is preferably 90% or more.
Glass transition temperature of the reinforcing plate is preferably 100xc2x0 C. or higher.
Furthermore, according to the present invention, a liquid crystal display device including the above mentioned backlight unit arranged on a rear surface of a liquid crystal display panel is provided.
In a case where the backlight unit is of the light-up type to be used below the liquid crystal display panel, the optical sheet is a light diffusing optical sheet for increasing the uniformity of luminance of the light-emitting surface of the backlight unit. The light source may be arranged immediately below the reinforcing plate and between the reflecting member and the reinforcing plate. In such case, the light source preferably comprises a plurality of juxtaposed line-shaped light sources and the reflecting member is preferably constructed with a support plate and a white reflecting plate supported by the support plate.
According to another aspect of the present invention, the backlight unit comprising a flat light guide panel for randomly reflecting incident light, which is emitted from a line light source arranged on a side edge of the flat light guide panel, within the flat light guide panel, a reflecting plate provided on one of major surfaces of the light guide panel, for reflecting light toward the side of the other major surface, a light diffusing optical sheet provided on the side of the other major surface of the light guide panel, for improving uniformity of luminance of a light emitting surface of the backlight unit, a light collecting optical sheet for improving luminance of a front surface of the backlight unit, a housing for housing the flat light guide panel, the reflecting plate, the light diffusing optical sheet and the light collecting optical sheet is featured by further comprising a transparent reinforcing plate in intimate contact with at least one of the light diffusing optical sheet and the light collecting optical sheet.
According to a further aspect of the present invention, the backlight unit comprising at least one line-shaped light source, a white reflecting plate for reflecting light emitted backward from the line-shaped light source and a light diffusing optical sheet for improving uniformity of luminance of a light emitting surface of light directly emitted from the line-shaped light source or reflected light by the white reflecting plate, is featured by further comprising a transparent reinforcing plate provided on a rear surface of the light diffusing optical sheet, which is a surface opposite to a beaded surface of the light diffusing optical sheet.
The reinforcing plate is formed of polyethylene naphthalate or polycarbonate having mechanical strength higher than that of the light diffusing optical sheet.
According to another aspect of the present invention, a non-light emitting type display device, which comprising a backlight unit and a liquid crystal display provided on the backlight unit and displays an image with light emitted from the backlight unit, is featured by that the backlight unit is the above mentioned back light.